1. Field of the Invention
The invention concerns frequency modulation broadcast transmitter synchronization methods. Synchronizing two transmitters guarantees that the signals output by all the transmitters are identical except for their level and a constant time-delay.
The invention is more particularly concerned with a method of synchronizing a plurality of transmitters in a broadcast network comprising a program production site connected by transmission links to transmitters which are remote from the production site which transmits to each transmitter a baseband source signal representing the program, each transmitter broadcasting a final frequency modulation signal derived from the source signal by a number of processing steps.
2. Description of the Prior Art
In a network of frequency modulation transmitters broadcasting the same sinusoidal carrier (the same radio program, for example) the problem arises of mutual interference between the different transmitters, especially in transmission overlap areas in which the field levels are not very dissimilar and which are regarded as critical areas because reception quality is very poor. This problem is essentially due to the fact that, because of their varying distances from the production site, the transmitters do not receive the same source signal at the same time, given the analog nature of the transmitted signal and the propagation time required to transmit it from the production site to each transmitter; consequently, the transmitters do not transmit the same final signal at a given time. This problem is accentuated because, depending on their distance from adjacent transmitters, the critical areas do not receive the same signals at the same time, because of the propagation time needed to transmit the signal from a transmitter to the critical area. One solution to this problem is to use different transmission frequencies for each transmitter to cover the critical areas. This leads to high frequency usage, however, and the need for a mobile listener periodically to retune his receiver to the frequency of the transmitter offering the best reception conditions, in order to stay with the same program.
An experimental radio broadcast network developed by the Italian broadcasting authority RAI uses a network of synchronized transmitters. The production site is linked to each transmitter by a monomode optical fiber which transmits a signal modulated at the final transmission frequency, the modulated signal being obtained from a single modulator encoder located at the production site. The transmitters receive the same modulated signal and amplify it before it is broadcast. In this way the signals output by the transmitters are synchronized, each transmitter receiving at its input the same signal with a transmission delay which substantially compensates the broadcast delay provided that the broadcast direction is identical to the transmission direction. This solution has many drawbacks, however:
it is incompatible with existing broadcast network structures,
it requires the use of a monomode optical fiber, in other words a costly infrastructure which is costly to install,
it uses only a negligible part of the transmission capacity of the transmission medium,
it requires a broadcast direction identical to the transmission direction.
The objective of the invention is to alleviate the drawbacks of the prior art and in particular to provide a network of synchronized frequency modulation transmitters using the conventional broadcast network structure, enabling simple and accurate adjustment of the phase alignment of synchronized signals at critical points in the service area, using equipment compatible with existing equipment enabling configuration and operation of the broadcast network in synchronized or non-synchronized mode, and in which the transmitters broadcast simultaneously a final signal at the same carrier frequency.